The invention disclosed herein pertains to an electronic controller for controlling a multiple electric power level load. The controller is especially useful for controlling an electromagnetically operated contactor that is characterized by having a magnetically attractable armature associated with a high current coil that pulls in the armature to close an electric circuit and a low current relatively high impedance holding coil that holds the armature in pulled-in condition.
Contactors of this general type are often used, together with mechanical, slow-acting switches, to energize or connect to a power source an electric motor that drives a refrigeration compressor. An example of a critical usage is in association with a refrigeration system on a semi-trailer truck where the power for operating the contactor is derived from the battery of the tractor that hauls the trailer, or a battery located on the trailer itself. As is known, the battery has a nominal or rated output voltage of 12 volts. The voltage can be much lower when the battery is partially discharged or the load on the battery other than the refrigeration compressor load is high. It is of critical importance having the refrigeration motor controlling contactor pull in when the thermostat calls for cooling, even if the voltage of the electrical system is substantially below nominal voltage. Failure of the contactor to pull in at low voltage can result in no refrigeration in the trailer and spoilage of a load of perishables.
In addition, failure of the contactor to pull in at low voltage can result in current being continuously passed through the pull-in coil without the armature closing and opening the normally-closed switch to the high current coil, with the undesirable result that the coil can be damaged or the battery further discharged.
This invention relates to improvements to the above apparatus and to solutions to some of the problems raised or not solved thereby.